1. Field of the Invention
The present invention relates to a dual phase stainless steel, and specifically relates to bonding dual phase stainless steel tubes for oil production, for corrosion resistance of chemical plant, for conveyer of crude oil, for heat exchanger of sea water conversion and relates to bonding dual phase stainless steel tube against a dual phase stainless steel flange for the above noted applications.
2. Description of the Related Art
A ferritic/austenitic dual phase stainless steel (dual phase stainless hereunder) is made of increasing content of Cr, of decreasing Ni and of adding Mo etc., and by being solution treated to make dual phase characteristics of austenitic and of ferritic. The dual phase stainless has good strength and toughness, and has good quality of stress corrosion cracking proof of inter-granular corrosion proof in comparison with the conventional austenitic stainless steel. Accordingly, it has wide application for oil environment and heat exchanger of sea water conversion.
Oil reservoir of oil well stays thousands meters underneath the ground. Dual phase stainless steel tube for tubing is used by way of connecting each piece of 10 to 15 meter length and putting them to down hole. As a method of connection, there exist way of mechanical coupling, orbital welding and supposedly liquid phase bonding.
The mechanical coupling method, as is shown in FIG. 4, makes screw connection by putting together fixed side of tubes (tube 1) which has been connected as a string of tube underground and the other side of upper movable piece (tube 2) by using external screw connector 7 which combines outer threads of down side tube 1, by its internal threads, and outer threads of upper side tube. The bonding face of the down side tube 1a and of upper side tube 2a contact together so that tube 1 and tube 2 is connected.
Mechanical coupling requires five to ten minutes to make one couple and has a good productivity at connecting but it has the disadvantage of allowing oil and gas leakage at the connection. Therefore special care for precision machining on to external threads of tube 1 and tube 2, and to connector 7 have been requested together with demand of high level skill of connection. The connected part has good tensile strength but may have problems in compression stress which may spreads out in a radiant direction from connector 7 and may accelerate leakage of oil and gas. As is obvious, connector 7 has a larger outer diameter than that of tubes 1 and 2 therefore, larger hole than outer diameter of tube 1 and 2 is basically demanded.
In the welding method, as shown in FIG. 5, at the top of tube 1 and bottom of tube 2, welding edges 1b and 2b are prepared and are placed together, and a welding torch 9 moves around the tube surface in circle to mount a melted metal 10 on edges 1b and 2b. This is a way of welding the tube 1 and tube 2.
Welding is advantageous because it does not cause leakage of oil and gas at the welded joint and has good compression resistance well as good tensile strength, plus, because a connector is not used, the same hole diameter as of the tube could be possible. However, in the welding method, the amount of welding can not be so large. Accordingly, particularly, in the case of welding thick tubes, problems which are encountered include consuming time as long as one to two hours to perform the welding, and talented skill is also prerequisite.
On the other hand, liquid phase diffusion bonding is a method as follows; Putting an insert material between two steel tube cross sections, applying pressuring power longitudinally, heating up and holding the temperature higher than the melting point of the insert material and lower than that of the tube for some time period to have some part of chemical composition spread in atomic particle to the both sides tubes and melting point of the residual of insert material, after moving atoms be evacuated, comes down which means the residual of insert will solidify itself by shifting melting point higher, in partly evacuated chemistry of insert, than used to be.
Liquid phase diffusion bonding has similarity in those characteristics which have no concern of oil and gas leak, compression resistance and keeping the same bonded joint diameter as of external tube diameter, plus higher productivity than welding in processing one batch in about 30 minutes as well as high quality and efficiency. Therefore liquid phase diffusion bonding is the most recommendable bonding method for joining oil tubular.
Liquid phase diffusion bonding has been well known technology. An example is Unexamined Japanese Patent Publication (kokai) No. Hei. 6-007967, in which a way of bonding high alloy tube for oil industry is described, putting insert material between two pieces of high alloy tubes, applying pressure of 4.9 MPa through 19.6 MPa (0.5 kg/mm.sup.2 to 2.0 kg/mm.sup.2) and heating up to 1200 through 1280.degree. C. and keeping the temperature longer than 120 seconds, when melting point of insert material is equal to or below 1150.degree. C.
Further, WO97/36711 discloses a method for bonding a stainless steel in the following manner. In this method, a martensitic stainless steel or a dual phase stainless steel including not less than 9 wt % of Cr is used. A low melting point bonding material composed of a Ni-based alloy foil including not less than 50 wt % of Cr, having the melting point of not more than 1150.degree. C. and having the thickness of 10 to 80 .mu.m is put between the bonding cross section faces of the stainless steels to be bonded. The pressure of 0.5 to 2 kgf/mm.sup.2 is applied while a temperature is kept between the melting point of the bonding material and that of the stainless steel for more than 120 second so that the heating length which becomes more than 800.degree. C. is 3 to 20 mm.
Unnecessary excessive heating causes troubles in mother material strength and in corrosion resistance. Therefore, high frequency induction heating has been popular for its partial and concentrated heat efficiency. In high frequency induction heating, because of surface effect, alternate electricity accumulates on tube surface and conducts to the inside of tube.
As described in Unexamined Japanese Patent Publication (kokai) No. Hei. 6-7967, bonding at relatively low temperature as low as below 1280.degree. C. and in short time heat keeping by high frequency induction will make unbonded boundary partially because of inefficient heat elevation and spread at cross section boundary.
On the other hand, by making heating time longer, such an inefficient bonding could be improved, but too long time heating mother material will deteriorate it and increase the deformation at boundary, which would be a good reason of concentration of stress and in case of internal deformation, be an obstacle for smooth liquid flow.
Furthermore in offshore oil field tubular, unexpected impact force may hit the tube. There has been no intention regarding how to improve impact resistant value at bonded area.
Moreover, referring to the method disclosed in WO97/36711, the present inventors conducted the bonding test of the dual phase stainless steel (diameter: 200 mm, thickness: 20 mm) having a practical size. The following conditions were selected so as to corresponding to those of WO97/36711.
As the material to be bonded, used was a dual phase stainless steel (JIS G3459; SUS329J1, melting point: 1420.degree. C.) including 23.0 to 28.0 wt % of Cr. As the low melting point bonding material, used was a nickel brazing material (JIS Z3265; BNi-5, melting point: 1080 to 1135.degree. C.) including 18.0 to 19.5 wt % of Cr and having the thickness of 40 .mu.m. The bonding temperature was 1300.degree. C., time for keeping the bonding temperature was 300 second, and the applied force was 1.25 kgf/mm.sup.2. Thus, the bonding test was conducted.
However, the bonded body which was obtained under the above-described conditions was extremely deformed at the bonding area, and the deformation amount of the bonding area exceeded 20 mm. Further, many cracks were observed at the surface of the bonding area.
Next, the test was conducted again in the same condition as described above except that the bonding temperature was changed to 1200.degree. C. which is less than the melting point of the nickel brazing material. In this case, the deformation amount of the bonding area of the bonding body thus obtained is a little, and no crack was observed in the surface of the bonding area, thereby capable of obtaining a good coupling in view of the external appearance.
From thus obtained dual phase stainless steel bonding coupling, four test pieces for the tensile test (length: 300 mm, width: 25.4 mm) was cut, and the tensile test was conducted. As the result of the test, all of the four test pieces were broken at the bonded boundary, and the tensile strength of them exhibited extremely low value as 200 to 400 MPa. Accordingly, in case of applying the conditions of WO97/36711 to the dual phase stainless steel having a practical size, it was not possible to obtain a good bonding coupling for practical use.